The present invention relates to a diffraction device, such as a spectroscope or a diffraction grating, for use in a spectral composition separating device, such as a demultiplexer, that separates an electromagnetic wave in accordance with differences in frequency.
When periodically dividing the wave front of an electromagnetic wave for light or the like, it is well known that phase differences produce diffraction waves. Thus, a diffraction grating for periodically dividing the wave front of an electromagnetic wave is widely used in spectroscopes or a diffraction lenses. In recent years, rapid progress has been made in the field of optical communication with technology for dense wavelength division multiplexing (DWDM). In this field, a diffraction grating is widely used since signals having very close wavelengths must be split or coupled. In addition to having a high resolution, the demultiplexer and multiplexer must be compact and inexpensive.
When dividing light having different frequencies with a diffraction grating, the resolution λ0/Δλ is represented as λ0/Δλ=mN. In the equation, λ0 represents the center wavelength, Δλ represents the resolved wavelength difference, m represents the order of the diffraction wave, and N represents the number of steps (refer to “Applied Optics” Tadao Tsuruta, Baifuukan 1990).
As apparent from the above equations, there are two ways to increase resolution, one of which is increasing the number of steps and the other of which is using higher order diffraction wave.
The lower limit of the diffraction grating period is about the same as the wavelength. Thus, increasing the number of steps to improve resolution enlarges the device size and makes the production of a flawless diffraction grating difficult. Accordingly, the diffraction grating is formed in a blazed profile to strengthen one high order diffraction wave. Further, there is a device with an increased order diffraction wave, or a device that uses a diffraction wave having a higher order, such as an arrayed waveguide grating (AWG). In an AWG, a wave front is divided and guided to different waveguides. The waveguides have different lengths to form an extremely large optical path length difference. This enables diffraction wave having a high order, such as 30th order, to be easily obtained.
In DWDM, wavelengths having, for example, a center wavelength λ0 of 1550 nm and an interval of Δλ=0.8 nm (the frequency interval being 100 GHz) must be separated. In such a case, λ0/Δλ=1937.5=mN is required. However, when using this mN value, a large amount of cross talk is produced. Thus, in the actual demultiplexer, mN=8000 is required to sufficiently decrease the cross talk between adjacent frequencies.
To achieve mN=8000, the number of steps is 8000 for a 1st order diffraction grating, and a diffraction grating having the size of 16 mm is required even if the grating period is 2 μm, for an AWG using 32 degree diffraction wave, the optical path length difference between adjacent waveguides is 1.55×32=49.6 (μm). The number of steps is 8000/32=250. Thus, the total optical path length difference reaches 49.6×250=12400 (μm). To ensure such optical path length difference, the size of the AWG reaches 20 to 30 mm.
When a diffraction device, such as an AWG, is enlarged, manufacturing becomes difficult, manufacturing costs increase, and characteristic changes resulting from temperature increase.
Accordingly, it is an object of the present invention to provide a diffraction device that decreases the value of resolution (λ0/Δλ), which corresponds to the difference between the separated frequencies, and uses a photonic crystal that reduces the size or improves the resolution.